docs: def correlation(s, o):
      """
          correlation coefficient
          input:
          s: simulated
          o: observed
          output:
          correlation: correlation coefficient
          """
      # s,o = filter_nan(s,o)
      if s.size == 0:
          corr = np.NaN
      else:
          corr = np.corrcoef(o, s)[0, 1]
      return corr